The present invention relates generally to a measurement method and apparatus, and particularly to a measurement method and apparatus for measuring an optical characteristic of a projection optical system that projects a pattern of a reticle (mask) to a substrate.
A projection exposure apparatus has so far been employed which uses the lithography technology to manufacture fine semiconductor devices such as a semiconductor device, e.g., an IC and an LSI, an imaging device, e.g., a CCD, a display device, e.g., a liquid crystal panel, a magnetic head. A projection exposure apparatus transfers a pattern of a reticle (mask) onto a substrate such a wafer via a projection optical system. Since the exposure apparatus is required to precisely transfer a pattern of a reticle to a substrate with a specified magnification, it is important to use a projection optical system that has an excellent imaging characteristic and a restrained aberration. Especially in recent years, finer processing to the semiconductor device progresses, and a pattern to be transferred has become sensitive to an aberration of an optical system. Accordingly, there is a demand to highly precisely measure a projection optical system's optical characteristic (e.g., a wavefront aberration) with the projection optical system included in an exposure apparatus. In addition, in order to improve productivity and economic efficiency, a simple, fast, and inexpensive measurement is also important.
Conventionally, a projection optical system's wavefront aberration has been measured by actually exposing a reticle pattern onto a wafer, and observing its resist image using such a means as a scanning electron microscope (“SEM”). This conventional measurement method has a problem in a poor reproducibility of measurement due to a difficult SEM operation and errors in a resist application and a development.
In order to rapidly and accurately measure a projection optical system's wavefront aberration, it is desirable to use an interference method, rather than using the conventional measurement method that exposes a pattern on a resist for evaluation. However, the conventional interference method that uses a Fizeau interferometer, a Twyman-Green interferometer, or the like makes an overall system's structure complex, thus implying a large-size and high-cost problem. Thus, it is difficult to mount the interferometer on an exposure apparatus, and the conventional interference method is not viable.
Therefore, an exposure apparatus is proposed that has a comparatively simple interferometer such as a point diffraction interferometer (hereinafter called a “PDI”), a line diffraction interferometer (hereinafter called an “LDI”), and the like. For example, see Japanese Patent Application, Publication No. 2004-273748.
The wavefront measurement using the LDI obtains a pair of primary wavefronts of the projection optical system in the measurement direction that is a direction perpendicular to a slit's longer direction, and calculates a wavefront of the projection optical system based on the pair of primary wavefronts. The “primary wavefront,” as used herein, is a wavefront having wavefront aberration information of the target optical system or projection optical system only in the measurement direction.
In measuring the first and second primary wavefronts, one major error cause of the wavefront to be measured is a difference of a position in the height direction (referred to as “Z position” hereinafter) of a measurement pattern on the image side to the exposure apparatus. More specifically, this is an error of a 2θ component, such as a mountain (valley) in a first measurement direction, and a valley (mountain) in a second measurement direction, in the wavefront measurement using the LDI and two primary wavefronts whose measurement directions are orthogonal to each other. Equation 1 below defines an error amount E2θ of the 2θ component, where dz is a Z position difference of the measurement pattern on the image side between the first and second primary wavefront measurements, λ is a wavelength of an exposure light source, and NA is a numerical aperture of a projection optical system:E2θ=dz×(1−sqrt(1−(NA)2)/2×λ)  EQUATION 1
For example, the error amount E2θ is 1.5 mλ for dz=1 nm in the exposure apparatus having λ of 193 nm and NA of 0.9. Control in order of several nanometers is arduous even with an apparatus mounted with a highly precise stage, such as an exposure apparatus. Conceivably, a measurement error of several mλ or greater occurs in the wavefront measurement using the LDI, causing the error in the wavefront measurement using the LDI.
The wavefront measurement using the LDI utilizes a wavefront of the diffracted light from the slit (slit diffracted wavefront) for a reference wavefront. The manufacturing errors of the slit, such as a slit width, a thickness of a light blocking part, the parallelism in the longer direction, and the roughness of an edge cause a difference between slits for the slit diffracted wavefronts, and this difference triggers the error in the wavefront measurement using the LDI.